With the progression of portability and cordlessness of electronic equipments, as their driving power supply, compact and lightweight non-aqueous electrolyte secondary batteries having high energy density, particularly lithium ion secondary batteries, are attracting attention. A typical lithium ion secondary battery is provided with: a positive electrode including a layered compound such as a lithium cobalt oxide; and a negative electrode including a material capable of absorbing and desorbing lithium such as graphite. Between the positive electrode and the negative electrode, a separator is interposed, which serves to electrically insulate these as well as retain a non-aqueous electrolyte. For the non-aqueous electrolyte, a non-aqueous solvent containing a lithium salt dissolved therein is used. In a number of aqueous secondary batteries, a battery voltage is no greater than 2 V, whereas lithium-ion secondary batteries have a battery voltage of about 4 V and also a high energy density. Thus, lithium-ion secondary batteries are used in a number of portable equipments.
In nickel-cadmium batteries and nickel-metal hydride batteries, a constant current charging method in which charging is performed at a constant current, a constant voltage charging method in which charging is performed at a constant voltage, or a −Δ V charging method in which charging is performed while also detecting reduction in charging voltage associated with oxygen absorption, is mainstream. On the other hand, in lithium-ion secondary batteries, a constant current-constant voltage charging method which is a combination of the above, or a pulse charging method, is performed.
In the constant current-constant voltage charging method, charging is performed at a constant current until the battery voltage becomes an upper limit value, and subsequently, charging is performed at a constant voltage until the current value is reduced to a predetermined value. In the pulse charging method, charging is performed by alternately repeating quick pulse charging and charging pause.
If a battery voltage is detected during charging in the pulse charging method, a difference occurs in the detected voltage due to the influence of a pulse current. For this reason, there may be difficulties in recognizing the charging condition based on the detected value of the battery voltage. Therefore, detecting the battery voltage for a plural number of times during charging pause and then calculating the precise battery voltage from the detected values, is being proposed (Refer to Patent Document 1).
On the other hand, in order to improve the charge-discharge cycle life, controlling the frequency of pulse charging is being proposed. For example, measuring an alternating current impedance of a battery in advance, and then charging at an arbitrary frequency at which the discrepancy of the phase relative to the applied voltage is within the range of −5 to 0 degrees, is being proposed (Refer to Patent Document 2).
The charging method as described in Patent Document 1 is a method in which the battery voltage is detected for a plural number of times during the pause period in pulse charging, the detected values are then calculated, and the calculated results are fed back to charging. For such a calculation, an A/D converter and a dedicated circuit in order to convert the battery voltage to digital signals are required. In addition, it is required to provide a pause time for voltage detection, thereby requiring a charging time longer than the charging time ordinarily required.
In addition, according to an example in Patent Document 1, the charging method is a method in which constant current charging is performed at the initial stage of charging when the battery voltage is low, and is then switched to pulse charging at a point when the battery voltage reaches a predetermined value. In such a charging method, because constant current charging is performed at the initial stage of charging, it is considered that cycle characteristics improve. However, in the case where constant current charging is performed until the battery voltage reaches about 4.2 V, the amount of electricity actually charged by the pulse current becomes small. For this reason, a period of time same as, or longer than that of the constant current-constant voltage charging, will be required.    [Patent Document 1] Japanese Patent Publication No. 3291402    [Patent Document 2] Japanese Laid-Open Patent Publication No. 2000-133320